Revisions log for Answer #1434

In your case, it's complicated by the fact that you're using OSB as your flooring. This completely envelopes your top floor (also acts as a vapor barrier). Especially if the vapor barrier underneath – as you wrote – was not done correctly, there is a significant risk to the structure. Moisture penetrates from below, but can no longer escape upstairs.

Because of the complexity (OSB, KWL) I can not give specific advice here, but imho, at least the flooring should be replaced with Rauspund (diffusion-open). Cut "strips" out of the boards or drill large holes in them, if necessary. But please consult an independent expert – who will also be responsible for his recommendations in this case.

But just to be clear: I see a dire need for action here. It can last for decades, but it can also cause the ceiling joists to rot within a very short time. Personally, the risk would be too great for me.